The thing about GF having no actual friends is undoubtedly why she's separated your son from his, @workworkworkugh - she's unable to comprehend the basic relationship of it all. The shared experiences, the comaraderie, the fact that when you have close friends, you can turn to them when you need a little bolstering or a shoulder to lean/cry on.
As I've said before, my daughter (24) has a diagnosis of NPD with traits of EUPD thrown in for "good" measure. When she was your son and the girlfriend's age, she didn't have any friends. Those she hung around with at school, openly reviled her when it came to parties and trips into town. As a mother, it was heartbreaking to watch her upset, yet at the same time, also because I knew why. It was entirely her own doing. When she started dating, the first boyfriend (at school) was seeing her for a dare, the second actually ghosted her, the third ran away to the other end of the country, and the fourth...? Well, I fully suspect she's lied to him about her illness and about me (she likes to claim that I'm abusive towards her - when actually, it's the other way around), so we've met once, briefly, and I was threatened with not telling him the truth about her. Every boyfriend she has, she decides she's going to marry after a few weeks of dating - this one, she's planning on buying a 4-bedroom house, marrying, and refusing to allow his small child to even visit "her" (ie, my daughter's) home. The boyfriend is pretty weak-willed and from what I can gather, very much enables my daughter's delusions of "happily ever after". She is also doing her utmost to separate him from his mates, by spending every free moment with him - and she complains bitterly when he sees his own daughter for 4 hours a week. I know it's not going to end well. I suspect your son's girlfriends parents know that their relationship isn't going to end well.
But end, it will, at one point or another. And you're doing the absolute right thing. I'm still flabbergasted by the lack of attention/support this girl's parents are actually paying to her/her life - the father truly believes that two 16 year olds aren't having sex?! It's remarkable... but also might explain a bit as to why her behaviour is so repulsive. If her own parents aren't paying her the attention that she believes she deserves - she will seek it elsewhere. And even bad attention, as we know from having raised toddlers, is attention at the end of the day. Perhaps she sees, in you, OP, the parents she actually craves her own to be? Hence the steady incline of threats/bad behaviour and then the mild apologies which you know she doesn't really mean.
Stand firm. Because as the mother of a PD girl, myself, I truly wish the latest boyfriend's parents were more like you, rather than buying into my daughter's fantasies as to how cruel and unusual I've "always been" towards her.